1. Field of the Invention
This invention relates to a disk cassette, specifically to a disk cassette having a shutter where the shutter is shaped into a U-form in section.
2. Description of the Related Art
As disclosed in Japanese Utility Model Registration Publication No. 60-6938, a disk cassette, such as a floppy disk cassette or the like is constituted as illustrated in FIGS. 9 through 11. A thin case 3 shaped substantially into a square-form comprises an upper half 1 and a lower half 2, both halves being made of synthetic resin. Rotatably accommodated in the case 3 is a floppy disk 4 that is a disk-like recording medium, and a pair of upper and lower head insertion openings 5, each of which is shaped substantially into a rectangular-form, are biasedly provided near one side edge 3a of the case 3. Further, a circular turntable insertion opening 6 is disposed in the central portion of the lower half 2.
A thin rectangular shutter 9 comprising a pair of upper and lower shutter plates 8 and a connecting plate 7, the shutter being U-form in section, is secured to cover the outside of the case 3 in the vicinity of the side edge 3a thereof. The upper and lower shutter plates 8 are fitted into respective stepped portions 10 formed in the upper and lower surfaces of the case 3, and the connecting plate 7 is located to be movable along the side edge 3a of the case 3. Thus, the shutter 9 can be moved in a direction of arrows a, b between a closed position, indicated in solid lines and an opened position, indicated in chain-dotted lines in FIG. 9.
When the shutter 9 is slid into the closed position, a pair of upper and lower openings 11 shaped substantially into a rectangular-form and provided in the respective upper and lower shutter plates 8 are moved to a position discrepant from the head insertion opening 5, so that the pair of head insertion openings 5 are closed by the pair of shutter plates 8, respectively. When the shutter 9 is slid into the opened position, openings 11 coincide with the head insertion openings 5, so that the head insertion openings 5 are opened. It is noted that the shutter 9 is located in the closed position due to force of a spring (not shown) which is accommodated in the case 3 to urge the shutter 9 in the direction of arrow a.
There are at least two kinds of shutters 9: one is made of a stainless steel sheet by the help of press working, and another is made of synthetic resin. Further, there are at least two kinds of production methods to make the shutter 9 of the synthetic resin.
One method of producing the shutter 9 of the synthetic resin is such that a synthetic resin plate 13 (FIG. 6) having a thickness T of less than 0.3 mm and made of polypropylene, vinylchloride or the like is press-worked, then horizontally mounted on a first jig 14 as shown in FIG. 6. After sandwiching the central portion of the plate 13 between the first jig 14 and a heated second jig 15, both end portions of the plate 13 are bent in a direction of arrow c with a pair of third jigs 16 as shown in FIG. 7, in order to obtain a shutter that is of U-shape in section.
Another method of producing the shutter 9 of the synthetic resin is such that a molten synthetic resin, such as polyacetal, polypropylene, polyethylene or the like is injected into a U-shaped cavity 19 through an injection gate 20, shown in FIG. 8, to make a shutter that is of U-shape in section. It is noted that the cavity 19 is formed between metallic molds 17, 18 and has a gap G of less than 0.3 mm.
However, if the shutter 9 is made by means of bending the synthetic resin sheet 13 while the synthetic resin sheet 13 is heated, residual stresses are produced, after bending, in corners 9a shown in FIG. 7, where the respective shutter plates 8 are connected to the connecting plate 7. This is because the outer side of the corner 9a is cooled at high speed, whereas the inner side thereof is cooled at low speed.
If the shutter 9 is made by means of injecting the synthetic resin into the metallic mold 17, 18 as shown in FIG. 8, residual stresses are also produced in the corners 9a after molding, because the molten synthetic resin 21 flows at high speed and turns sharply along the corner portion 19a of the cavity 19, and further, the molten synthetic resin 21 that fills the corner portion 19a cools more quickly than the synthetic resin that fills plain portions 19b, 19c of the cavity 19.
The residual stresses existing in the corner 9a of the shutter 9 act as a force for transferring the shutter plates 8 in a direction of arrow d in FIG. 7, so that if the disk cassette is used at high temperature, the shutter plates 8 of the shutter 9 are easily deformed in the direction of arrow d as indicated in dot-dash lines in FIG. 10. As the result, sometimes the disk cassette is difficult to draw from a recording and/or reproducing apparatus, and sometimes the shutter drops off of the case 3.